Investigators involved in modern electronic and electro-mechanical industries increasingly have sought more refined and efficient devices and techniques in the generation of motion and the effectuation of its control. For example, the mass storage of data for computer installations is carried out by recordation on magnetic disks which are rotatably driven under exacting specifications. The speed of data handling performance of these computation systems is very much dependent upon the available speed of recording and retrieval from the surfaces of the disks. Similar requirements of performance are to be observed in the fields of robotics, machine tools and the like.
In the past, pneumatic and hydraulic movers were resorted to for a great many of the latter applications. However, a significant trend to electrically powered devices has occurred because of their inherently faster and more reliable motion control. Such control capability has greatly expanded with the emergence of the microprocessor on a significant scale.
Permanent magnet (P.M.) direct current (d.c.) motors represent the largest and most cost effective portion of the current electrical motor market. These motors occur in a wide variety of designs.
Generally, the classic P.M. d.c. motor is a three-phase device having a stator functioning to mount two or more permanent magnet poles which perform in conjunction with three of more rotor mounted field windings which are wound over the inward portions of pole structures typically formed of laminated steel sheets. The ends or tip portions of the rotor poles are flared or curved somewhat broadly to improve their magnetic interaction with the stator magnets. Typically, field windings are intercoupled in either a delta or Y circuit configuration and by exciting them in a particular sequence, an electromagnetic field, in effect, is caused to move from one flared pole tip to the next to achieve an interaction with the permanent magnets and evolve rotational motion. This interaction occurs in almost all designs through an air gap which is disposed "radially" to the motor shaft between the stator and rotor in parallel relationship with the axis of the rotor, i.e. a "radial gap" is provided. The interaction between the permanent magnet field and the field of the excited windings is one wherein force vectors are developed in consequence of an association of the exciting field with the field or flux sense of the magnets. Where typical ferrite or alnico type permanent magnets are used, any other disposition of the field interaction would effect a destructive demagnetization of these magnets. Classically, the switching providing select excitation of the field windings is provided by a commutator rotating with the rotor and associated with brushes representing a make and break mechanical switching device functioning to move the field along the pole tips.
As the classic P.M. d.c. motor has been applied to more sophisticated electromechanical systems, it has been found to be dificient in many aspects. For example, the make and break commutation is electrically noisy, a condition which in many applications will be found to be unacceptable. The motors are heavy and are large and these aspects contribute to undesired design requirements for bulk where the designer loses much of the flexibility of innovation which is desired. While the motors have been produced in great quantities, their production is hindered by the nature of the pole structure carrying their field windings. Because of the flared ending or curved tips of the individual poles carrying the windings, the procedure for effecting winding is one somewhat complex and must be carried out underneath the flared tips on a fully assembled rotor. This requirement has impeded design progress which otherwise would be realized with motor structures which are simply changed to alter performance characteristics.
To address the performance limitation of electrical noise caused by the brush type motors, brushless P.M. d.c. motors have been developed wherein field commutation otherwise carried out mechanically has been replaced with an electronic circuit. These motors generally provide a higher quality performance including a much quieter electrical performance. However, this quiet electrical system, wherein the magnetic components move as opposed to the field windings, to date, has been implemented in relatively larger sizes than otherwise desired. Further, the windings, as in earlier motors, are provided beneath flared pole tips on the inside of the stator surface and thus are even more difficult to assemble and are not amenable to simple alteration for customized manufacture and the like.
Another characteristic of typical d.c. motors having poles configured as steel cores with associated field windings resides in a somewhat inherent development of detent torques. At rest, or in a static state, the steel poles of a typical rotor will assume an orientation with respect to associated permanent magnets which develops flux paths of highest density and least reluctance. Thus, were one to hand rotate the rotor of an unenergized motor, these positions of rest or detent positions can be felt or tactilly detected as well as the magnetic field induced retardation and acceleration developed in the vicinity of the detent positions. During an ensuing excitation state of the motor windings, this detnet torque will be additively and subtractively superimposed upon the operational characteristic of the motor output to develop instantaneous speed variations (ISV) which are generally uncorrectable, for example, by electronics. ISV characteristics also can be generated from mechanical unbalance phenomena in the rotor of a motor itself or the bearings thereof if they are a part of the rotating mass. Additionally, the effects of the above-described detent torque contribution to ISV can be somewhat amplified where the characteristics tend to distort the otherwise idealized torque characteristic curve which normally will exhibit a form of ripple often accommodated for by the addition of more phases to the architecture. Where these characteristics of distortion occur, the result can be quite pronounced at lower motor speeds. In the past, the output of the motors has been smoothed through resort to rotational masses such as flywheels and the like, however, for great numbers of modern applications, motors exhibiting large ISV characteristics are unacceptable. For this reason, spindle motors for floppy disk drives have been configured as vector cross products or B cross I devices, sometimes known as voice coil motors, which do not employ steel pole structures. Another approach which has been employed has been to alter the axially aligned gap of the motors to a twisted orientation by developing a step form of association of the sheets of steel forming the laminated steel pole cores. Of course, this leads to further complexity in the design of the motors and in the manufacture of them with internally manufactured field windings.